Resistive touch screens are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers.
FIG. 3 shows a portion of a prior-art resistive touch screen 10 of the type shown in Published US Patent Application No. 2002/0094660A1, filed by Getz et al., Sep. 17, 2001, and published Jul. 18, 2002, which includes a rigid transparent substrate 12, having a first conductive layer 14. A flexible transparent cover sheet 16 includes a second conductive layer 18 that is physically separated from the first conductive layer 14 by spacer dots 20 formed on the second conductive layer 18 by screen printing
Referring to FIG. 4, when the flexible transparent cover sheet 16 is deformed, for example by finger 13 pressure, to cause the first and second conductive layers to come into electrical contact, a voltage applied across the conductive layers 14 and 18 results in a flow of current proportional to the location of the contact. The conductive layers 14 and 18 have a resistance selected to optimize power usage and position sensing accuracy. The magnitude of this current is measured through connectors (not shown) connected to metal conductive patterns (not shown) formed on the edges of conductive layers 18 and 14 to locate the position of the deforming object.
Alternatively, it is known to form the spacer dots 20 for example by spraying through a mask or pneumatically sputtering small diameter transparent glass or polymer particles, as described in U.S. Pat. No. 5,062,198 issued to Sun, Nov. 5, 1991. The transparent glass or polymer particles are typically 45 microns in diameter or less and mixed with a transparent polymer adhesive in a volatile solvent before application. This process is relatively complex and expensive and the use of an additional material such as an adhesive can be expected to diminish the clarity of the touch screen. Such prior art spacer dots are limited in materials selections to polymers that can be manufactured into small beads or UV coated from monomers.
It is also known to use photolithography to form the spacer dots 20. In these prior art methods, the spacer dots may come loose and move around within the device, thereby causing unintended or inconsistent actuations. Furthermore, contact between the conductive layers 14 and 18 is not possible where the spacer dots are located, thereby reducing the accuracy of the touch screen, and stress at the locations of the spacer dots can cause device failure after a number of actuations. Unless steps are taken to adjust the index of refraction of the spacer dots, they can also be visible to a user, thereby reducing the quality of a display located behind the touch screen.
The conductive layers, 14 and 18 have stringent performance requirements. A typical sheet resistivity requirement is between 100 and 600 ohms per square, uniformity must be very high, and durability to over a million actuations is required. A transparency in excess of 85% with no coloration is also required for many applications. Various methods of providing the conductive layers 14 and 18 are known in the prior art, for example the use of indium tin oxide (ITO) or conductive polymers such as polythiophene. However, ITO tends to crack when stressed and conductive polymers may lack physical robustness and durability when stressed. Moreover, when ITO is employed the thickness of the coating required to provide adequate durability limits the transparency and resistivity.
U.S. Pat. No. 4,220,815 (Gibson et al.) and US Patent Application US20040090426 (Bourdelais et al.) disclose integral spacer dots on flexible cover sheets for touch screen applications. However, integral spacer dots must not have their top surfaces coated with the conductive layer to avoid electrical shorts between the first and second conductive layers, 14 and 18. US20040090426 addresses such need by high energy treatment (corona discharge treatment or glow discharge treatment) of the peaks of the spacer beads to provide surface energy difference to allow for differential surface wetting of an applied conductive layer, or by scraping of an applied conductive layer from the peaks of the spacer dots. In U.S. Pat. No. 4,220,815, cover sheet is provided with insulator islands created by deforming the cover sheet against a resilient surface with a punch. The force exerted by the punch destroys the conductive layer coated on the other side of the cover sheet. Each insulating island is associated with a corresponding dimple in the upper surface of cover sheet. Such requirements add complexity to the manufacturing process, and may negatively impact yields. Further, these approaches may not adequately electrically isolate the insulating islands, and will have reduced lifetime due to stresses induced in the cover sheet. Moreover, the dimples on the back side of the cover sheet are objectionable or, if filled, require additional materials and manufacturing steps to fill.
There is a need therefore for an improved means to form and separate the conductive layers of a touch screen and a method of making the same that improves the robustness and performance of the touch screen and reduces the cost of manufacture.